Display panel conversion data deciding method and measuring apparatus

ABSTRACT

A high-speed correction of display panel luminance variation is performed by use of a display panel conversion data deciding method, which comprises a first measuring step for determining a first driving current of the light emitting elements of the display panel when the capacitors of the pixels other than pixels to be measured have not completely been discharged; a charging step for charging, by an analog voltage, the capacitors of the pixels to be measured; a second measuring step for determining a first driving current of the light emitting elements of the display panel when the capacitors of the pixels to be measured have been charged by the analog voltage; a driving current calculating step for determining, from the difference between the first and second driving currents, the driving current of the pixels to be measured; and a data calculating step for determining conversion data based on the driving current.

1. FIELD OF THE INVENTION

The present invention relates to a method for determining the conversiondata of a display panel, particularly a method for determining theluminance conversion data for correcting variations in luminance of aTFT array display panel having self-emitting elements, and a displaydevice that uses this method.

2. DISCUSSION OF THE BACKGROUND ART

Flat display panels used in flat-screen televisions, monitors ofpersonal computers, display devices of portable telephones, and the likemust be capable of responding to fast-moving images and of vivid colorreproduction. In light of such demands, attention has recently beenfocused on thin film transistor (TFT) arrays with a fast response speedand active display panels that use organic EL elements and otherself-emitting elements with a wide range of display colors.

Self-emitting elements are emission elements that generate light inaccordance with the amount of current flowing to the element. A currentthat is much larger than that in a TFT array for a liquid crystal panel,which is a conventional flat display panel, must flow through a TFTarray used in a display panel with this type of self-emitting elements.When the amorphous silicon film that has been used for years in liquidcrystal display panels is employed in TFT arrays for display panels withself-emitting elements, it is often the case that an insufficientdriving current is obtained because the carrier mobility is low.Moreover, the variations in luminance of each pixel increase as thethreshold voltage of the FET changes over time as a result of chargebuild-up inside the gate insulation film. Therefore, a low-temperaturepolysilicon film, with which a high driving current is easily obtainedbecause of high carrier mobility and there are few changes over time, isoften used in TFT arrays of display panels with self-emitting elements.Nevertheless, when low-temperature polysilicon film is used, thecurrent-voltage properties of each FET change by approximately 10%depending on the extent to which crystals form in the FET channelregion. Moreover, this change can vary greatly, even among FETs that areclose together inside a panel. That is, there are large fluctuations inthe luminance of each pixel during production of TFT arrays that uselow-temperature polysilicon film. In addition, changes over time in thelight emission properties of a light-emitting element itself cannot bedisregarded. In particular, EL elements use organic materials;therefore, the extent of changes over time varies considerably with thetemperature, driving current, and other conditions under which anelement is used. Such fluctuations in emission luminance are a source ofdisplay panel defects manifested as image irregularities and colorchanges.

Therefore, it is necessary to measure the fluctuations in emissionluminance of each pixel and correct those fluctuations as necessaryduring the production and the use of conventional display panels thatuse self-emitting elements. The device in JP (Kokai) [Unexamined]5[1993]-80101 is a device for measuring and correcting the luminance ofa display panel. By means of this device, a test pattern is read by asensor located inside or outside a liquid crystal display panel, thelight output properties of the display panel are measured, and thecorrected data are renewed.

In addition, the technology disclosed in JP (Kokai) [Unexamined]2002-40074 is a technological means for measuring the driving current ofan EL element and evaluating defects in an EL display panel. That is,this is technology whereby the precise driving current of a pixel to bemeasured is found and defects in a display panel are evaluated from thedifference in the driving current by measuring, as shown in FIG. 1, thedriving current of a light-emitting element 115 after completelydischarging a charged capacitor 130 of a pixel 117 of a display panelcomprising a pixel selection transistor 131 for selecting pixels,capacitor 130, a drive transistor 118 for passing a driving current thatis in accordance with the voltage of capacitor 130, and self-emittingelement (EL element) 115, such as an EL display panel 108.

By means of the above-mentioned method, the next pixel must be measuredonce the driving current of a pixel under test has been measured andthen the capacitor of that pixel under test is completely discharged,that is, discharged to the threshold value of the drive transistor orlower; therefore, considerable time between pixel measurements is neededin order to continuously measure pixels. Moreover, an EL element itselfhas a capacitance component 143 and an impedance component 141, as shownby the equivalent circuit in FIG. 6. Therefore, once application of thedriving current has been initiated, it takes the amount of timecorresponding to a time constant to reach the steady state (state whenthe driving current is virtually constant). Consequently, there is aproblem with continuous measurement of the many pixels in a displaypanel in that such measurement takes a very long time.

However, one property of human vision is that differences in theluminance between pixels that are close to one another are noticed asimage irregularities and changes in color, but differences in theluminance of pixels that are not close to one another are not noticed.That is, the difference in relative luminance between pixels that areclose to one another should be measured in order to correct fluctuationsin luminance. Consequently, there is a need for a measurement methodthat is simpler and faster than conventional methods because absolutemeasurement in order to correct fluctuations in luminance is notnecessary.

SUMMARY OF THE INVENTION

The present invention solves the above-mentioned problems with a methodfor determining the conversion data of a display panel, characterized inthat it is a method for determining the conversion data of a displaydevice having a display panel, wherein there are disposed, in matrixform, multiple pixels, each having a capacitor, a drive circuit forcontrolling current or voltage based on the voltage of the capacitor,and a self-emitting element driven by the drive circuit, and a luminancesignal generating means for applying to the capacitor an analog voltageobtained by conversion of the luminance data based on conversion data,and in that it comprises a first measurement step for finding a firstdriving current of the light-emitting elements of the display panel whenthe capacitors of the pixels other than the pixel under test have notbeen completely discharged; a charging step for charging the capacitorof the pixel under test to the analog voltage; a second measurement stepfor measuring a second driving current of the light-emitting elements ofthe display panel when the capacitor of the pixel under test has beencharged to the analog voltage; a driving current calculation step forfinding the driving current of the pixel under test from the differencebetween the first driving current and the second driving current; and adata calculation step for finding the conversion data based on thedriving current.

That is, even if there is a pixel present in the display panel in whichthe capacitor has not been sufficiently discharged prior to the test, itis possible to cancel the driving current of that pixel and performhigh-speed measurement of variations in properties between pixels byusing a method whereby the driving current of light-emitting elements ofa display panel are measured before a pixel under test is measured andthe driving current of the light-emitting elements of the pixel undertest is found based on the difference from the driving current of thelight-emitting elements of the display panel when the pixel under testhas been driven. Furthermore, measurement at an even higher speed ispossible by measuring, before the light-emitting elements are driven,every predetermined number of pixels and interpolating from themeasurement results the current before driving unmeasured pixels. Inthis case, there are fluctuations in the properties of each pixel andthe precise pre-driving current therefore cannot be found byinterpolation, but because the absolute fluctuations are small inproportion to the amount of discharge, the effect of the variationsbetween adjacent pixels can be disregarded.

Moreover, the present invention solves the above-mentioned problems witha method for determining the conversion data of a display panel,characterized in that it is a method for determining the conversion dataof a display panel having a display panel comprising a TFT array andself-emitting elements, a luminance signal generating means forgenerating luminance signals by converting luminance data to conversiondata, a drive means for driving the self-emitting elements by theluminance signals, and a measurement means for measuring the drivingcurrent and/or emission luminance of the light-emitting elements of theTFT array, and in comprising a step for driving the self-emittingelement of the pixel under test, a step for performing the measurementbefore the driving current of the pixel under test has reached asaturated state, and a step for determining the conversion data based onthe results of the measurement. That is, measurement at an even higherspeed is possible by performing the measurement before the emissionluminance or driving current of the pixel under test reaches a saturatedstate (the emission luminance or measurement current reach the steadystate when an element is driven).

The present invention makes possible the high-speed correction ofvariations in luminance of a display panel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of an example of the measuring apparatus of thepresent invention.

FIG. 2 is a drawing that shows the measurement points of the example.

FIG. 3 is a drawing that shows another version of the measurementpoints.

FIG. 4 is an explanatory drawing of measurement luminance.

FIG. 5 is a drawing that shows a method for controlling a luminancesensor.

FIG. 6 is a drawing that shows an equivalent circuit of an EL element.

FIG. 7 is a drawing that shows the conversion data of a luminance signalgenerating circuit.

FIG. 8 is a drawing that shows a method for determining conversion data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the display device of the present inventionwill now be described in detail while referring to the attached drawing.EL elements are used as the self-emitting elements in these examples,but the present invention is not limited to an EL display panel and canbe used on display panels that use other self-emitting elements, such asa display panel that uses light-emitting diodes.

FIG. 1 is a structural diagram of the display device of the presentinvention. The display device comprises a control part 100 of the paneland an EL display panel 108. Control part 100 comprises: a selectionmeans in the form of a pixel selection circuit 104 connected to shiftregisters 109 and 110 of EL display panel 108; a luminance signalgenerating circuit 102, which is connected to the outside input ofluminance data and a luminance signal line 112 of EL display panel 108,and provides the conversion data of each pixel; a measurement means inthe form of an ammeter 101; a drive means in the form of a power source103 connected through ammeter 101 to a common line 119; and a conversiondata determination means in the form of a data processor 105, which isconnected to ammeter 101 and has a memory and a data processing circuit.As shown in FIG. 7, luminance signal generating circuit 102 has aconversion table in which luminance data 10 corresponding to lowluminance and luminance data 250 corresponding to high luminance arestored for every pixel (displayed by column number and row number).

Moreover, EL display panel 108 comprises multiple pixels disposed inmatrix form; a data line 111 and a gate line 116 for selecting pixels;and shift registers 109 and 110 connected to data line 111 and gate line116, respectively. A pixel 117 comprises a pixel selection transistor Q1131 connected to data line 111 and gate line 116; a capacitor C1 130connected to pixel selection transistor 131 and a common line 119; an ELelement 115; and a drive transistor Q2 118 connected to capacitor 130,pixel selection transistor 131, and EL element 115. A constant-currentcircuit is used as the drive circuit in the present example, but avoltage control circuit can also be used.

The operation of the display device in FIG. 1 will now be described. Thedisplay device has a normal display mode and a corrected mode. First, inthe normal display mode, pixel selection means 104 outputs pixellocation signals in accordance with pixel signals (pixel position dataand luminance data) that have been input from the outside, and shiftregisters 109 and 110 select the data line and the gate linecorresponding to the pixel position. For instance, when gate line 116and data line 111 are selected, pixel 117 located at the point ofintersection is selected. Similarly, luminance signal generating circuit102 calculates the analog voltage corresponding to the input luminancedata from the conversion data (luminance data 10 and luminance data 250)corresponding to each pixel and feeds that voltage to luminance signalline 112. For instance, when the luminance data that has been input is150, the voltage is 2.17 V (=1+(3−1)/250−10)×(150−10)) for the pixel atcolumn=0 and row=0. The luminance signal of luminance signal line 112 issupplied to data line 111 that has been selected by pixel selectioncircuit 104. On the other hand, pixel selection transistor 131 is turnedon at selected pixel 117, capacitor 130 is charged by the luminancesignal on data line 111, and then the above-mentioned voltage ismaintained by turning off pixel selection transistor 131. The current ofdrive transistor 118, which is a constant-current circuit, is controlledby the voltage of capacitor 130, and a driving current is applied to ELelement 115. EL element 115 emits light in an amount corresponding tothe amount of driving current.

It should be noted that the luminance data in the present example arelimited to 0 and within a range of 10 to 250; therefore, convertedvalues of luminance data 10 and luminance data 250 are used asconversion data, but any luminance data can be used as the conversiondata, and it is possible to select from a range of numerical values forthe luminance data. Linear interpolation is used in the present example;therefore, luminance data should be selected that correspond to thelower limit and upper limit of a region wherein the driving current(which is proportional to the capacitor applied voltage) has linearproperties with respect to the luminance data as in FIG. 4, but it isalso possible to use a region having nonlinear properties when nonlinearinterpolation is used.

Next, the operation of the correction mode will be described. Adescription of the operation of the structural parts inside EL displaypanel 108 will be omitted because it is the same as for the normal mode.First, 0 V luminance signals are applied to luminance signal line 112,selection transistor 131 of each pixel is selected in succession bypixel selection circuit 104, and all capacitors 131 of EL display panel108 are initialized. Once initialization is completed, the currentflowing to ammeter 101 is stored in the memory of data processor 105.Next, pixel under test 117 to be measured is selected by pixel selectioncircuit 104. Analog voltage corresponding to luminance data 10 isapplied from luminance signal generating circuit 102 to luminance signalline 112 at this time. The current flowing to ammeter 101 is stored inthe memory of data processor 105 also at this time. The driving currentImin1 of pixel under test 117 can be found from the difference betweenthe current before and the current after EL element 115 has been driven,the values of which are stored in the memory. When Imin1 is only 80% ofthe pre-set Imin0 as in FIG. 8, the conversion data of luminance data 10of luminance signal generating circuit 102 is increased by 1.25 times(=1/0.8).

Next, luminance signal generating circuit 102 applies 0 V to luminancesignal line 112 and capacitor 130 discharges. It takes time untilcapacitor 130 is completely discharged, that is, until capacitor 130 isdischarged to the threshold voltage of transistor 118; therefore, pixelselection transistor 131 of the pixel in question is turned off beforethe capacitor is discharged to the threshold voltage and the samemeasurement is performed on the next pixel under test. A pre-determinedcurrent continues to flow to drive transistor 118 of pixel 117 under theresidual potential of capacitor 130 of pixel 117; therefore, the currentflowing to ammeter 101 is stored in the memory of data processor 105before the EL element of the next pixel under test is driven and thedriving current of the next pixel under test is found from thedifference between that current and the current when the EL element isdriven. Thus, high-speed determination of conversion data is possible bystarting the measurement of the next pixel under test before thecapacitor of the pixel under test has been completely discharged.

The panel is initialized once the measurement of luminance data 10 ofthe pixel requiring measurement has been completed. Moreover, themeasurement and conversion data are determined for luminance data 250 bythe same process. That is, as shown in FIG. 8, the driving current Imax1is found when luminance signals corresponding to luminance data 250 havebeen applied to capacitor 131, Imax1 is compared with pre-determinedcurrent value Imin1, and the conversion value of luminance data 250 ofluminance signal generating circuit 102 is revised. Thus, pixels havingthe properties shown by the solid line in FIG. 8 can be corrected torealize predetermined properties as represented by the broken line.

The measurement points of ammeter 101 in the present example are shownin FIG. 2. References 401, 402, 403, and 404 in the figure are thecurrents that flow to ammeter 101 before the driving current flows tothe EL elements of the pixel under test, and references 411, 412, 413,and 414 are the driving currents when the EL element of the pixel undertest has been driven. Once a pixel under test has been measured, thenext pixel is measured without completely discharging capacitor C1;therefore, the current that flows to ammeter 101 gradually increasesbefore the EL element of the pixel under test is driven.

The discharge properties of the capacitor vary from pixel to pixel, andthe increase in current is not precisely constant, but it should bepossible to maintain a measurement and correction accuracy that issufficient for measurement for correcting fluctuations in luminance andthe driving current; therefore, if the increase in current is regardedas constant, it will not pose any problems in terms of practical use.Consequently, the display device of the present example has a mode formeasuring the current before measurement for a certain number of pixelseach time without measuring the current before measurement of everypixel, linearly interpolating from the most recently measured drivingcurrent, and finding the current before measurement of the pixel undertest. When this mode is selected, for instance, driving current values402 and 403 are found by interpolation from the actual measurements ofdriving currents 401 and 404 during the step wherein the differencecomponent after driving current 401 has been measured until drivingcurrent 404 is measured is calculated by data processor 105 withoutactually measuring the driving current flowing to display panel 108before the EL element of the pixel under test is driven. Thus,high-speed determination of conversion data is possible by reducing thenumber of times the current is measured when a pixel under test is notdriven.

The display device comprises measurement means and conversion data meansin the present example. Therefore, pixels under test can be measured asneeded and fluctuations in the driving current can be corrected, notonly when a device is being made but also when it is being used. As aresult, it is not necessary to install variation correction means, suchas a current mirror circuit or another self-correcting circuit, for eachpixel 117 of display panel 108; therefore, the device structure can besimplified and an inexpensive device can be provided.

Moreover, control part 100 of the present example can be separated fromthe display device as an individual measuring apparatus. In this case,the display device comprises a luminance signal generating circuit 102,a power source 103, and a pixel selection circuit 104 that are used fornormal display, and the measuring apparatus comprises luminance signalgenerating circuit 102, power source 103, and pixel selection circuit104 that are used for determination of conversion data. The structureand operation of the measuring apparatus are the same as for theabove-mentioned correction mode, but it is necessary to transmit theconversion data that have been determined by measurement to theluminance signal generating circuit housed inside the display deviceconnected to the outside. Therefore, it is necessary to install anoutput device in luminance signal generating circuit 102 of themeasuring apparatus.

The method for finding the difference between the measurement before theEL circuit of a pixel under test is driven and the measurement when theEL circuit is being driven described above can also be used as themethod for directly measuring luminance only as shown in JP (Kokai)[Unexamined] 5[1993]-80101. FIG. 5 is a drawing showing a sketch of aluminance measuring apparatus added to the display device of the presentexample. A luminance sensor 121 that scans EL display panel 108; aluminance detection circuit 122 that is connected to luminance sensor121 and detects luminance from the output signals from a sensor 121, anda sensor control circuit 123 that controls the operation of sensor 121are added to the device structure in FIG. 1. A light-blocking means 120is set up around sensor 121 and sensor 121 is constructed such that itcan detect only the light from pixels adjacent to the pixel under test.

The operation of the device that also measures luminance will now bedescribed. Operations other than measurement of luminance are the sameas for the above-mentioned device and the value is a description istherefore omitted. First, sensor control circuit 121 moves sensor 121 tothe pixel under test. Luminance is measured before pixel under test 117is driven and this value is stored in the memory of data processor 105.Next, EL element 115 of pixel under test 117 is driven by the drivingcurrent corresponding to luminance data 10 and luminance data 250, theluminance when the element is driven is measured, and the conversiondata of luminance signal generating circuit 102 are corrected. Moreover,capacitor 130 of pixel under test 117 is discharged and the next pixelis measured in succession before the capacitor is completely discharged.

As shown in FIG. 3, it is possible to determine conversion data at aneven higher speed by measuring the driving current or emission luminanceof each pixel before the driving current of emission luminance of thepixel under test reaches the steady state and a specific time after thedriving current is first applied. In this case, the precise drivingcurrent and emission luminance in the steady state cannot be measured,but there is a proportional relationship between the driving current andthe emission luminance at a specific time after current is first appliedand the driving current and emission luminance in the steady state;therefore, the conversion data can be corrected using measurements whenin a state of transition.

It should be noted that the present embodiment and modified examplesthereof are only one embodiment for describing the present invention ascited in the claims and persons skilled in the art will recognize that avariety of modifications are possible within the scope of the claims.

1. A method for determining the conversion data of a display panel,characterized in that it is a method for determining the conversion dataof a display device having a display panel, wherein there are disposedin matrix form multiple pixels, each having a capacitor, a drive circuitfor controlling current or voltage based on the voltage of thecapacitor, and a self-emitting element driven by the drive circuit, anda luminance signal generating means for applying to the capacitor ananalog voltage obtained by conversion of luminance data based onconversion data, comprises the steps of: a first measurement step forfinding a first driving current of the light-emitting elements of thedisplay panel when the capacitors of the pixels other than the pixelunder test have not been completely discharged; a charging step forcharging the capacitor of the pixel under test to the analog voltage; asecond measurement step for measuring a second driving current of thelight-emitting elements of the display panel when the capacitor of thepixel under test has been charged to the analog voltage; a drivingcurrent calculation step for finding the driving current of the pixelunder test from the difference between the first driving current and thesecond driving current; and a data calculation step for finding theconversion data based on the driving current.
 2. The method of claim 1,wherein the self-emitting element is an EL element.
 3. The method ofclaim 1, wherein the first measurement step is executed every time aspecific number of pixels is measured and; the first driving current ofthe pixel under test is found by interpolation from the driving currentactually measured in the first measurement step immediately before andimmediately after the pixel under test.
 4. A display device comprises: adisplay panel, wherein there are disposed in matrix form multiplepixels, each having a capacitor, a drive circuit for controlling currentor voltage based on the voltage of the capacitor, and a self-emittingelement driven by the drive circuit; a selection means for selecting anypixel under test; a luminance signal generating means for applying tothe capacitor an analog voltage obtained by conversion of luminance databased on conversion data; a measurement means for measuring the drivingcurrent of the light-emitting elements of the multiple pixels; and aconversion data determination means for finding the conversion databased on the difference between the first driving current of thelight-emitting elements of the multiple pixels when the capacitors ofpixels other than the pixel under test have not been completelydischarged and the second driving current of the light-emitting elementsof the multiple pixels when the capacitor of the pixel under test hasbeen charged to the analog voltage.
 5. A measuring apparatus of adisplay panel, characterized in that it is a measuring apparatus of adisplay panel wherein there are disposed in matrix form multiple pixels,each having a capacitor, a drive circuit for controlling current orvoltage based on the voltage of the capacitor, and a self-emittingelement driven by the drive circuit, comprises: a selection means forselecting any pixel under test, a luminance signal generating means forapplying to the capacitor an analog voltage obtained by conversion ofluminance data based on conversion data; a measurement means formeasuring the driving current of the light-emitting elements of themultiple pixels; a conversion data determination means for finding theconversion data based on the difference between the first drivingcurrent of the light-emitting elements of the multiple pixels when thecapacitors of pixels other than the pixel under test have not beencompletely discharged and the second driving current of light-emittingelements of multiple pixels when the capacitor of the pixel under testhas been charged to the analog voltage; and an output means foroutputting the conversion data.
 6. A method for determining conversiondata of a display panel, characterized in that it is a method fordetermining the conversion data of a display device having a displaypanel, wherein there are disposed in matrix form multiple pixels, eachhaving a capacitor, a drive circuit for controlling current or voltagebased on the voltage of the capacitor, and a self-emitting elementdriven by the drive circuit, and a luminance signal generating means forapplying to the capacitor an analog voltage obtained by conversion ofluminance data based on conversion data, comprises the steps of: a firstmeasurement step for finding the first emission luminance of the displaypanel when the capacitors of pixels other than the pixel under test havenot been completely discharged; a charging step for charging thecapacitor of the pixel under test to the analog voltage, a secondmeasurement step for measuring the second emission luminance of thedisplay panel when the capacitor of the pixel under test has beencharged to the analog voltage; an emission luminance calculation stepfor finding the emission luminance of the pixel under test from thedifference between the first emission luminance and the second emissionluminance; and a data calculation step for finding the conversion databased on the emission luminance.
 7. A method for determining theconversion data of a display panel, having a display panel comprising aTFT array and self-emitting elements; a luminance signal generatingmeans for generating luminance signals by converting luminance data toconversion data; a drive means for driving the self-emitting elements bythe luminance signals, and a measurement means for measuring the drivingcurrent and/or emission luminance of the light-emitting elements of theTFT array, comprises the steps of: a step for driving the self-emittingelement of the pixel under test, a step for performing the measurementbefore the driving current or emission luminance of the pixel under testhas reached a steady state; and a step for determining the conversiondata based on the results of the measurement.